1. Field of the Invention
This invention relates to sintered alloys and a method for the hardening treatment thereof. More particularly, it relates a treatment method which permits iron-based sintered articles to be hardened either wholly or locally and which can be applied especially to synchronizer hubs and like components for use in the transmissions of four-wheeled vehicles.
2. Description of the Related Art
Roughly speaking, the following two methods for improving the wear resistance of an iron-based sintered alloy have been known in the prior art.
(1) A first method is one for hardening the whole of a sintered alloy or one for hardening a sliding part of a sintered alloy by infiltrating an impregnant into the whole of the sliding part.
Specific examples thereof include (a) a method for the hardening treatment of a compact in which, in the formation of a compact by compressing an iron-based sinterable powder, the composition of the whole compact is modified by increasing the contents of C, Cr, Mo, V and other elements constituting the sinterable powder, (b) a method for hardening a sintered alloy by a heat treatment such as quenching or tempering, and (c) a method which comprises infiltrating a copper impregnant into a sintered alloy as disclosed in Japanese Patent Provisional Publication No. 61-44152.
(2) Another method is one for hardening a portion of a sintered alloy or one for hardening a sliding part of a sintered alloy by infiltrating an impregnant into a portion of the sliding part.
Specific examples thereof include (a) a heat treatment method based on induction hardening, and (b) a method which comprises compacting and sintering a sinterable powder while varying the composition of the sinterable powder locally so that the composition of some parts of the compact will differ from that of other parts.
However, the above-described conventional methods for the hardening treatment of a sintered alloy have the following problems.
(1) In the methods based on a heat treatment, a compact is heated again after being sintered, so that the sintered alloy tends to become distorted. Consequently, the sintered ally requires an additional examination of its dimensions and an additional adjustment of its dimensional accuracy, which tends to increase the number of steps.
(2) In the methods which involve varying the composition of the sinterable powder, the powder control for changing the sinterable powder according to the type of the sintered alloy causes an increase in the number of steps. Moreover, since it is necessary to set particular sintering conditions for each sintered alloy, various types of sintered alloys cannot be produced simultaneously.
(3) The methods which involves infiltrating a copper impregnant into a sintered alloy within a sintering furnace are efficient from the viewpoint of thermal energy and production control. However, difficulties are encountered in treating a sintered alloy locally. Moreover, its hardness cannot be increased to the fullest extent and, therefore, its wear resistance has a certain limit.
By way of example, for synchronizer hubs for use in the transmission gears of four-wheeled vehicles, iron-based sintered articles are often used from the viewpoint of uniaxial configuration and strength. However, in the as-sintered state, these articles have low hardness and hence exhibit insufficient wear resistance. Although they may be subjected to a heat treatment such as induction quenching after sintering, this requires additional thermal energy and causes an increase in manufacturing cost.
Accordingly, in order to solve the above-described problems, it would be necessary to apply a hardening treatment to only a sliding part of a sintered article instead of the whole of the sintered article, and carry out this hardening treatment at the same time as the sintering.
An object of the present invention is to solve the above-described problems by providing sintered alloys in which only a sliding part can be hardened without altering the dimensional accuracy of the article, and a method for the hardening treatment thereof.
In order to accomplish the above object, the present invention includes the following three embodiments.
(Embodiment 1) First of all, the present invention relates to a method for the hardening treatment of a sintered alloy which comprises the steps of compressing an iron-based sinterable material to form a compact; providing a surface of the compact with a coating material containing aluminum or an aluminum alloy that melts at a temperature lower than the sintering temperature of the compact; and sintering the compact provided with the coating material, so as to form an intermetallic compound of iron and aluminum in a surface layer of the compact.
According to the above-described method, a hardening treatment for forming an intermetallic compound can be applied to not only the whole of a sintered alloy, but also only a part of a sintered alloy (e.g., a part thereof required for use as a sliding site). Moreover, the present invention can eliminate the necessity for an after-treatment such as quenching, and thereby save labor. Consequently, an enhancement in the efficiency of thermal energy required for sintering and an improvement in the dimensional accuracy of the resulting sintered alloy can be achieved. Furthermore, even when a compact is to be hardened locally, it can be treated under the same conditions as employed commonly for sintering purposes, without adding to an amount of work required for the hardening treatment.
(Embodiment 2) Moreover, the present invention relates to a method for the hardening treatment of a sintered alloy in accordance with the above-described Embodiment 1 wherein the coating material comprises a dispersion of a powder of aluminum or an aluminum alloy in a solvent or a member formed of aluminum or an aluminum alloy.
According to the above-described method, while the compact is sintered to form a sintered alloy, a portion of the aluminum component present in the coating material penetrates from the surface of the compact into a plurality of pores formed in the compact. On the other hand, the iron component present in the compact also penetrates into the coating material. Consequently, a reaction takes place between the aluminum component of the coating material and the iron component of the compact, so that an intermetallic compound is formed in a surface layer of the sintered alloy. This intermetallic compound has very high hardness and wear resistance, and hence exhibits characteristic suitable for use as a sliding part. Moreover, since the intermetallic compound is a porous body having a multitude of pores or the like, it allows oil and the like to accumulate therein and hence serves to reduce the sliding resistance.
(Embodiment 3) Furthermore, the present invention relates to a sintered alloy having been subjected to a hardening treatment according to the method of the above-described Embodiment 1 or 2.
The present invention makes it possible to apply a hardening treatment to not only the whole of a sintered alloy, but also only a part of a sintered alloy (e.g., a part thereof required for use as a sliding site). Moreover, the present invention can eliminate the necessity for an after-treatment such as quenching, and thereby save labor.